1. Field of the Invention
This invention relates to an in-focus detecting device suited to be used, for example, in an image pickup optical system of the video camera.
2. Description of the Related Art
The video cameras or the like have generally employed the device for detecting the focus of the optical system for the image pickup device by measuring the degree of image sharpness. To perform focusing of the optical system, for example, the video signals obtained from the image pickup element are processed to detect the degree of sharpness of an object image on the image pickup surface, and the optical system is moved into focus in such a way that the degree of sharpness becomes the highest. For this purpose, many methods have been proposed, the typical one of which is fundamentally in that the high frequency component of the video signal is extracted by the high pass filter or the differentiation circuit, and the degree of sharpness of the image is evaluated by the magnitude of the extracted high frequency component. When focusing is performed by such a method, the direction of driving of the optical system is determined by comparing the degrees of sharpness of two images of different focus states from each other obtained by having driven the opticald system, and the optical system is stopped in a position where the degree of sharpness has reached the maximum. Thus, the in-focus condition of the optical system is obtained.
FIG. 14 illustrates an example of the construction of the conventional in-focus detecting device employing such a method. In this figure, 1 is an image forming lens for forming an image of an object to be photographed on an image pickup plane; 2 is a drive circuit for driving an image pickup element 3; 4 is a gate circuit for defining a distance measuring area in the image pickup plane; 5 is a band pass filter for detecting a prescribed frequency band in the distance measuring area; 6 is a drive circuit for causing minute vibrations of the image forming lens 1 within its depth of focus; 10 is an actuator for driving the image forming lens 1 to be driven by the drive circuit 6; 7 is a microcomputer receptive of the output signal of the band pass filter 5 for determining the direction of driving of the lens 1 and the in-focus position; 8 is a drive circuit for controlling the focus position of the image forming lens 1 on the basis of the command of the microcomputer 7; 9 is an actuator to be driven by the drive circuit 8.
FIG. 15 illustrates that, as an image of some object is picked up, the frequency component obtained from the band pass filter 5 in its prescribed area varies as a function of the position of the lens 1. An in-focus point C is the position of the lens 1 in which the frequency component takes the maximum value as the figure shows. The focusing device of FIG. 14 controls the movement of the lens 1 so that it stops in the corresponding position to this in-focus point C.
To detect the above-described maximum value, a lens constituting part of the image forming lens 1 is made to vibrate minutely back and forth with variation of its frequency component.
FIG. 16 illustrates the variation of the frequency component across the in-focus point. The waveform on line A of this figure represents a drive signal for minute vibration of the lens 1. The waveforms on lines B to D represent the outputs of the band pass filter 5 when the lens 1 is positioned on the front side of the in-focus point, at the in-focus point and on the rear side of the in-focus position, respectively. As shown by the waveforms of B and D in comparison, the phase of variation of the output of the band pass filter component changes 180.degree. across the in-focus point. Where this phase inverts becomes the in-focus point at which the output no longer changes as shown on line C of this figure. So, the microcomputer 7 accepts the amplitude and phase of the output signal of the band pass filter 5 to determine the direction in which the image forming lens 1 is to be moved and sends a command of driving and stopping the image forming lens 1 to and in the in-focus position C to the drive circuit 8 for the actuator 9.
However, though such a conventional method is effective in the neighborhood of the in-focus condition of the object, it is in a largely defocused position, for example, point E of FIG. 15, that the component of the output of the band pass filter 5 does not largely change with variation of the lens position as shown on line E of FIG. 16, becoming indistinguishable from the in-focus signal. Therefore, the conventional device employing this method has an important drawback that when the image is largely defocused, the direction in which the image forming lens 1 is to be moved to bring the image into sharp focus is uncertain, so that there is no way of driving except to rely on the rule of trial and error.